The Message Authenticator Algorithm is, as far as the author
is aware, the first Cryptographic Hash Function or Message Digest
to gain widespread acceptance. It has become a part of ISO
standard 8731-2: Approved Algorithms for Message
Authentication. The designers were Donald Davies and David
Clayden. As it seems to be unavailable in electronic form,
here's an on-line version and a
.pdf
version. See appended notes for extra material.

Caveat Encryptor

Electronics Weekly, April 15, 1998, page 2, reported:
'100Mips Amulet3 ready by year end'. Amulet3 is the
asynchronous version of the ARM7 Thumb core being developed at
Manchester University.&nbsp The crypto interest lies in its
asynchronous design. The time taken by each instruction is
variable and is data dependent. The up side is that EMI is
much reduced due to lack of clocks. However, designers who
want to use this chip for crypto algorithms should consider the
increased vulnerability to timing attacks and power monitoring
attacks. Both these attacks effectively reduce the keyspace
of the algorithms.

The use of Cyclic Redundancy Checks for message
authentication is asking for trouble. Here are some notes,
based on Mark Lomas' security lectures to Cambridge
undergraduates that show how to subvert a message,
hidden under a stream cipher, that is 'protected' by a CRC.

A talk given by Sir Harry
Hinsley to Cambridge University's Security Group on the
use of Ultra during WWII.

Lastly, articles on how 3 U-Boats were caught by deep laid mines off Padstow
as a result of Ultra intelligence.

Amateur Radio - Antennas

Antenna Matching

One of the perenial problems of Amateur Radio is that of antenna bandwidth.
The worst example of this in the HF bands is the 160 metre (1.81 to 2.00 MHz) band.
It's common to express antenna bandwidth as the band between the points where the SWR is
1.5 or less compared with the centre of the band. In the case of 160 metres this is 10%.

The antenna behaves like a tuned circuit and, when power is applied, the reactance of an off-tune
antenna causes power to be reflected back down the feeder (coax) back to the transmitter.
If the transmitter's final drive stage has no protection, as in the case of very old or cheap equipment,
then damage to the final drive stage is likely.

Faced with this problem in the 1980's an experiment was done that used stubs attached to the antenna
feedpoint. These were in multiples of a quarter wavelength. Odd multiples were closed-ended
stubs and even multiples were open-ended. Although stubs look like tuned circuits electrically,
the reactance changes in the opposite direction to that of a tuned circuit, so it can be used to
cancel the reactance of an antenna.

In the particular case tested, the results showed a dramatic improvement
in SWR figures. However, further analysis is needed - in particular to establish if
the improvement led to more power being tranmitted.

One Wavelength Loops

As a matter of curiosity a number of simulations were run, using EZNEC+, to look at the
properties of rectangular loop antennas with a perimeter
of one wavelength. These results are not normally shown in the antenna books and point
to a different approach to designing Quad/Yagi antennas that require good bandwidth.
For reference, the same runs were done on both dipoles with droopy ends and double loops.

Embedded Microprocessors - ADC Conversion without division

Small embedded microprocessors such as PICs and AVRs often have 10-bit ADCs
whose outputs need to be converted to recognisable units such as millivolts,
degC etc. Ideally, the conversion process should involve adds, multiplies
and shifts only.

One of the problems is due to the range of ADC values - 0 to 1023.
This means division by 1023 or a multiple of 1023 somewhere in the maths.
This can be circumvented by scaling up the equation by 1024/1023, simplifying
and then changing the remaining 1024/1023 factor to 1025/1024.

The difference between 1024/1023 and 1025/1024 is less than 1 ppm which
is an acceptable level of error. Here is
the math describing the process and
a spreadsheet to allow inspection of all
ADC output values and experimentation.

For those with a keen interest in espionage, here are some photos of a
dead letterbox.

Description of a computer illiterate:
Someone who can't tell his ASCII
from his EBCDIC.

Psychology

Sociopaths

Has your life ever been made miserable by a psychopath/sociopath?
Most people have suffered at the hands of a perverse personality
at sometime in their lives, be it in the workplace or at home.
It may take years before the lovable glib mask is penetrated and the
destructive side exposed. An estimated 1 in 25 of the population
are sociopaths - which means that we all know a few. Journalist John
Simpson's description of Robert Maxwell is a good
illustration.

In a book review on Psychopathy by Millon et al, the reviewer quotes:
". . . it was not until Cleckley wrote The Mask of Sanity in 1941
that we came to realize that psychopathy is a personality disorder that
wreaks havoc on personal and societal institutions. As reported in
chapter 8, Westman estimates that each sociopath costs society
about $50,000 a year." The case of
Robert Hendy-Freegard is one of the
worst ones to emerge recently.

This extended excerpt from a book by
Martha Stout gives a stark insight into the mindset of this type of
personality. When read slowly and carefully it can form the
basis for understanding the phenomenon. Notes include warnings
from religious texts as well as modern scientific findings involving
DNA markers and brain scans.

Groupthink

Have you ever fallen foul of a cosy clique of callous critics?
You may be facing a phenomenon known as
Groupthink
where a group of people who may seem OK on an individual basis act in
a perverse way in a group context. The group can range from the
local parish council up to a national government.

The inept handling of outgroups and indiviuals by the ingroup is
matched by the inept way the ingroup deals with its own situations.
This can only be remedied by positive dissent from within the ingroup.

Irving Janis,
a research psychologist from Yale University has written a number of books
on the subject such as Groupthink: psychological studies of policy
decisions and fiascoes where he dissects and compares situations
such as the Bay of Pigs fiasco with that of the Cuban Missile crisis.

A body of men holding themselves accountable to nobody ought not to be
trusted by anybody — Thomas Paine